The present invention relates generally to suspension systems for motor vehicles, and more particularly to an active fluid suspension system. Still more particularly, the invention deals with an improved device for controlling the active fluid suspension system for optimum performance characteristics at various vehicle speeds.
The active fluid suspension system has been known and used, which comprises a set of fluid suspension units disposed one at each vehicle wheel. Air or other fluid is introduced into and withdrawn from the suspension units under the control of an electronic controller in response to the output signals of sensors which detect a prescribed set of vehicle behaviors. Therefore, the suspension units function for leveling the vehicle, for the absorption of shocks and vibrations, and for other purposes. The active fluid suspension system contributes to the enhancement of the ride under all traveling conditions of the vehicle.
The pending Japanese Patent Application No. 62-107992, filed by the present applicant, discloses an improved active fluid suspension system. It suggests the use of what are termed "insensitivity zone filters" within the controller. These filters are intended to provide "insensitivity zones", such that the controller does not respond to those portions of the sensor output waveforms whose amplitudes are between given positive and negative values. The insensitivity zone filters serve to preclude unnecessary fluid flow into and out of the suspension units in response to the sensor output signal portions of negligibly small amplitudes. Thus the filters appreciably reduce the energy required for fluid flow and, at the same time, makes possible the more positive control of the suspension units.
However, this pending application has a weak point. If the insensitivity zones are determined for optimum suspension control in a low vehicle speed range, energy of vibrations which the vehicle receives from the road increases in proportion to the square of the vehicle speed. Consequently, at high vehicle speed, the sensor output signals become so great in amplitude that they mostly pass the insensitivity zone filters unsuppressed. The result is a drastic increase in the amounts of the fluid that must be unnecessarily forced into and withdrawn from the suspension units, running counter to the objective of economy in the energy used for such fluid flow.
It would provide no solution to this problem to determine the insensitivity zones so as to suit the higher vehicle speed range. For, in that case, the controller would fail to make the necessary control of the suspension units in the lower vehicle speed range.